The present invention relates to resin impregnated fiber composites and more particularly to a manufacturing process for controlling the temperature of vacuum resin infusion process by utilizing external fluids.
Fiber reinforced composite parts are fabricated utilizing a variety of conventional techniques including vacuum resin infusion, resin transfer molding (RTM), prepreg/autoclave procedures, and compression molding operations. Vacuum resin infusion consists of infusing a preform with liquid resin under vacuum using a one sided tool. Resin transfer molding differs from vacuum resin infusion by infusing the preform with liquid resin under pressure with or without vacuum using a matched, two sided tool capable of withstanding the pressure. Fiber reinforced plastic structures have been commercially produced for some years. Examples of manufacturing techniques can be found in U.S. Pat. No. 5,052,906 Entitled xe2x80x9cPlastic Transfer Molding Apparatus For The Production of Fiber Reinforced Plastic Structuresxe2x80x9d, U.S. Pat. No. 5,601,852 Entitled xe2x80x9cUnitary Vacuum Bag For Forming Fiber Reinforced Composite Articles And Process For Making Samexe2x80x9d, U.S. Pat. No. 4,132,755 Entitled xe2x80x9cProcess For Manufacturing Resin-Impregnated, Reinforced Articles Without The Presence Of Resin Fumesxe2x80x9d, U.S. Pat. No. 5,129,813 Entitled xe2x80x9cEmbossed Vacuum Bag, Methods For Producing And Using Said Bagxe2x80x9d, U.S. Pat. No. 4,902,215 Entitled xe2x80x9cPlastic Transfer Molding Techniques For The Production Of Fiber Reinforced Plastic Structuresxe2x80x9d, U.S. Pat. No. 4,942,013 Entitled xe2x80x9cVacuum Resin Impregnation Processxe2x80x9d, U.S. Pat. No. 5,439,635 Entitled xe2x80x9cUnitary Vacuum Bag For Forming Fiber Reinforced Composite Articles And Process For Making Samexe2x80x9d, U.S. Pat. No. 5,281,388 Entitled xe2x80x9cResin Impregnation Process For Producing A Resin-Fiber Compositexe2x80x9d, U.S. Pat. No. 5,316,462 Entitled xe2x80x9cUnitary Vacuum Bag For Forming Fiber Reinforced Composite Articles, and U.S. Pat. No. 2,913,036 Entitled Process And Apparatus For Molding Large plastic Structuresxe2x80x9d, all of which are hereby fully incorporated herein by reference.
The process for producing these structures requires the incorporation of a resin or other flowable plastic material into a reinforcing fiber. Reinforcing fiber generally takes the form of one or more layers of a woven or felted fiber reinforcement, typically comprised of carbon, graphite, or fiberglass. The vacuum resin infusion or impregnation process is usually done by either a wet or dry fiber lay-up technique. In the wet fiber lay-up process, the resin xe2x80x9cwettedxe2x80x9d fiber reinforcement consists of a prepreg which already contains a resin and is laid up on a mold and cured.
In the dry lay-up process, the fiber reinforcement is laid up dry on a mold or form which serves as a mold. The form may be incorporated as part and parcel of the finish product. Thereafter, the fiber is sprayed, brushed, impregnated, infused, or otherwise coated or xe2x80x9cwettedxe2x80x9d with the resin. The resin is then cured to form the fiber reinforced plastic structure.
During the curing stage of either process, the structure can be placed in a vacuum to assist the curing process. To this end, vacuum bag techniques have been used to provide such vacuum assistance. In a vacuum technique, a flexible impervious sheet, liner, or bag is used to cover a single mold which contains the dry or wet (resin impregnated) fiber lay-up. In the wet fiber process, the edges of the flexible sheet are clamped against the mold to form an envelope and seal the resin impregnated fiber lay-up to the mold and out of the atmosphere. A vacuum is then applied to consolidate the preform during the cure of the resin. In the dry fiber lay-up, catalyzed liquid plastic or resin is generally introduced into the envelope or bag interior to wet the dry fiber, usually using a vacuum (usually applied before resin introduction) to help push the resin into the bag and wet out the dry fiber. Vacuum is applied to the bag interior via a vacuum line to collapse flexible sheet against the fiber and surface of the mold, and then the plastic wetted fiber is processed, compacted and cured to form the fiber reinforced structure. The vacuum bag used in this process is critical because it provides a vacuum seal and consolidation pressure.
Prior vacuum resin infusion processes have had a number of problems when fabricating a large part. One of the problems is that the height of the part is limited by the maximum practical vacuum pressure of xe2x88x9214.7 psi=xe2x88x92xcfx81ghMAX (where g is gravitational acceleration). Using conventional methods, the maximum part height (hMAX) is about 33 ft., assuming a minimum resin density (xcfx81) of 1.03 grams per cm3. Methods to overcome this 33 ft. limitation, such as multi-stage infusions, may be undesirable because the stage boundaries are subject to property variations. Another technique to overcome the 33 ft. limitation is to use pressurized injection. This may be undesirable however because of the risk of inflating the bag, or the cost associated with rigid matched tools. Also, vacuum resin infusion processes have typically been slow proportionally as the effective vacuum resin infusion pressure decreases as a function of height. Slow filling increases the risk of resin gelation before completing the infusion. Therefore, the risk of having a bad part increases as a function of height, especially above 20 ft.
Also, part height limits processing options such as xe2x80x9cfinal vacuumxe2x80x9d. After filling with a high vacuum ( greater than 27xe2x80x3 Hg), it is desirable to reduce the final vacuum to minimize porosity and control resin content. If the final vacuum is too high, the volatiles within the resin are more likely to boil, creating undesirable porosity. Also, high vacuum tends to yield low resin contents, which may be undesirable for some applications.
Another problem with prior vacuum resin infusion processes is that the effective processing pressure varies as a function of height per the formula pgh. This pressure gradient yields property variations, such as resin content as a function of height. Resin content variations yield variations in strength, modulus, toughness and specific gravity of the final part and are therefore undesirable.
Another problem with prior vacuum resin infusion processes is that thick parts have heretofore been vulnerable to excessive exotherm temperatures, which may lead to undesirable side effects. Resin curing is an exothermic reaction and a large mass of resin within thick sections supplies reaction energy which increases temperature further accelerating the reaction. Therefore, thick sections generally have poor heat transfer mechanisms so the curing reaction can get quite hot ( greater than 20xc2x0 F.). Excessive heat has several undesirable side effects, such as the fact that it can decompose the laminate and seriously degrade mechanical and physical properties. Excessive heat can also create thermal shrinkage gradients, residual stresses and cracking. Also, excessive heat can boil volatiles (such as styrene) within the resin, and create excessive porosity which degrades mechanical and physical properties.
What is needed then is a technique for a vacuum resin infusion process which overcomes the deficiencies described above. Efforts in this area have led to continuing developments to improve their versatility, practicality and efficiency.
An object of the present invention is to provide a vacuum resin infusion process which utilizes fluid to control the temperature of the process.
According to the present invention, a method of manufacturing a resin impregnated reinforced article comprises the steps of providing a permeable reinforcing material on a mold, sealing the material with a flexible, impervious sheet, introducing resin into the material, drawing a vacuum on the material within the sealed sheet, controlling the temperature of the material and resin by placing a fluid in contact with the impervious sheet.
The present invention provides a cost effective vacuum resin infusion process which minimizes resin variations and the porosity of the finished part. The present invention also minimizes mechanical and physical degradation, thermal shrinkage gradients, residual stresses, and cracking. In addition, the present invention accelerates processing time.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.